Modern reliable computer systems require a large capacity mass storage, and large bandwidth access to that mass storage. While disk sizes have increased substantially, for example a typical personal computer today can be configured with over a gigabyte of storage, the bandwidth available to access the storage has improved, but not significantly. Thus, while large volumes of information can be stored, the rate at which the storage can be accessed has not generally changed in the past few years. In particular, considering the requirements of a digital video system for the delivery of constant bit rate video streams such as MPEG-2 digital video streams, it is important to provide a high bandwidth to accommodate the data requirements of the digital video network, for example an ATM OC-3 network interface.
While various methods have been employed to provide sufficient quantities of data in a reliable configuration, perhaps one of the more popular, and least expensive, approaches is the use of RAID-5 striping and parity techniques to organize data in a fault tolerant and efficient manner. The RAID (Redundant Array of Inexpensive Disks) approach is well described in the literature and has various levels of operation, including RAID-5, and the data organization can achieve data storage in a fault tolerant and load balanced manner.
In particular, RAID-5 provides that the stored data is spread among three or more disk drives, in a redundant manner, so that even if one of the disk drives fails, the data stored on the drive can be recovered in an efficient and error free manner from the other storage locations. This method also advantageously, using RAID-5 striping, makes use of each of the disk drives in relatively equal and substantially parallel operations. Accordingly, if one has a six gigabyte cluster volume which spans three disk drives, each disk drive would be responsible for servicing two gigabytes of the cluster volume. Each two gigabyte drive would be comprised of one-third redundant information, to provide the redundant, and thus fault tolerant, operation required for the RAID-5 approach.
Consider a processor reading a video data object from a local RAID-5 array. In normal operation, using a RAID-5 approach, when the processor needs to access a video data object which is spread across all of the disk drives connected to the processor, the processor reads a portion of the video data object in a round robin fashion from each of the disk drives. For example, a first 64 kilobyte block of the video data object can be stored and read on a first drive, the next 64 kilobyte block being stored on the second drive, and so on. In addition, however, the parity check (actually an EXCLUSIVE-OR function), also a 64 kilobyte block, is stored so that if there were n disk drives there would be one parity block written for each n−1 blocks of data.
The processor reading the disk drives, however, is still “stuck,” with a relatively narrow bandwidth. Accordingly, the amount of data which can be read is limited by the bus to which the drives are connected. For example, a SCSI bus which, while providing substantial improvements over buses from years ago, is still relatively slow compared to the needs of video applications. Also, the use of a local RAID-5 controller can combine the outputs of multiple local SCSI buses, but is subject to the failure of the local processor. Such a failure eliminates access to all the data.
Accordingly, objects of the invention are a method and apparatus having improved and increased mass storage read and write bandwidth (delivery bandwidth), operating using a reliable and fault tolerant protocol in a novel topology and enabling large quantities of data to be read and written in accordance with well known and accepted techniques. Other objects of the invention include a method and apparatus which is relatively inexpensive, reliable, simple to build, and easy to maintain.